A SERP preview renders a title, URL and description the way a search result displays them, so you can see the listing before it exists rather than after.
What it shows you
Where the title truncates. Whether the description reads as a complete thought or stops mid-sentence. How the two work together as a single unit of persuasion.
Why writing blind fails
Titles and descriptions are written in a CMS field that gives no indication of display width. A headline that reads perfectly in the editor can lose its last four words in the results page — and those are usually the words carrying the offer.
The habit worth building
Preview before publishing, not after a click-through rate report shows something is wrong.
Try it: Google SERP Snippet Preview on SeoWolf's Notepad.